Primary cortical neurons are nerve cells isolated directly from the cerebral cortex of an animal, most often from the embryonic brains of rats or mice. The term “primary” signifies that these cells are harvested directly from living tissue for use in research.
These cells are tools for neurobiology research, allowing scientists to study nerve cells in a controlled environment outside of a living animal. This approach makes it possible to investigate cellular behaviors and responses that would be difficult to observe in the brain itself.
The Role of Cortical Neurons in the Brain
The cerebral cortex, the brain’s outermost layer, is the hub for higher-order cognitive functions like memory, attention, perception, and thought. Cortical neurons are the functional units within this structure, carrying out the processes that underlie our ability to interact with the world. Their activity enables everything from processing sensory information to controlling voluntary movements.
Within the cortex, two main classes of neurons maintain a balanced interaction for proper brain function. Excitatory neurons make up about 80% of the cortical neuron population and act to increase electrical activity in the cells they connect to. The most common type is the pyramidal cell, which sends signals over long distances to other brain regions.
In contrast, inhibitory interneurons comprise the remaining 20% of cortical neurons. These cells decrease electrical activity, regulating and shaping the signals carried by excitatory neurons. The dynamic interplay between excitatory and inhibitory neurons creates the intricate circuits that allow for complex brain functions.
Isolation and Culture Process
Obtaining primary cortical neurons for research is a multi-step procedure performed under sterile conditions to prevent contamination. The process begins with dissecting the cerebral cortex from a rodent embryo. Embryonic tissue is chosen because the developing neurons adapt more readily to a culture environment than mature neurons. The timing is specific, using embryos from a particular day of gestation to ensure the cells are at the ideal developmental stage.
Once the cortical tissue is isolated, it is broken down to release individual cells through enzymatic and mechanical dissociation. First, enzymes digest the extracellular matrix that holds the cells together. The tissue is then gently broken apart mechanically, often by passing it through a series of pipettes, until a suspension of single cells is achieved.
The final step is to plate the dissociated neurons onto a prepared surface in a culture dish, which is coated with a substance like poly-D-lysine to help them adhere. The cells are bathed in a specialized growth medium containing nutrients and growth factors that support their survival. Over several days to weeks in an incubator, these neurons extend axons and dendrites, forming complex networks that resemble circuits in the living brain.
Primary Neurons vs. Immortalized Cell Lines
Researchers choose between primary neurons and immortalized cell lines based on a trade-off between biological realism and practicality. Primary neurons are a more physiologically relevant model because they retain the characteristics of neurons in a living brain. They develop complex morphologies, form functional synapses, and exhibit the same electrophysiological properties as their in-vivo counterparts.
Primary neurons have significant challenges. They are finite because they are post-mitotic, meaning they do not divide and cannot be propagated in culture. Each experiment requires a new isolation from animal tissue, which introduces variability. They are also delicate and require a specialized environment to survive for a few weeks.
Immortalized cell lines, such as the SH-SY5Y line, offer a convenient alternative. These cells are derived from tumors or genetically modified to proliferate indefinitely, creating a consistent supply. This makes them easier to grow and maintain for large-scale studies. The drawback is that these cells do not fully replicate the complex functions of healthy neurons, as their characteristics can be altered over time.
Applications in Neuroscience Research
Primary cortical neurons are used across a wide spectrum of neuroscience research, from discovery to therapeutic development. They provide an in-vitro system for modeling neurodegenerative diseases like Alzheimer’s. Researchers use these cultures to observe cellular processes in a controlled setting, investigating disease mechanisms like the formation of amyloid plaques or tau tangles and how they lead to neuronal death.
In neurotoxicology, scientists expose cultured neurons to substances like pharmaceuticals or environmental toxins to assess their impact on neuronal health. This screening can identify compounds that are harmful to the nervous system or those that may offer protection. Parameters such as cell viability, neurite outgrowth, and synaptic function are measured to quantify a compound’s effects.
Primary cortical neurons are also used in research into how the brain works. They are used to study synaptic plasticity, the process by which connections between neurons strengthen or weaken, which is the cellular basis for learning and memory. By stimulating or silencing specific neurons in a culture, scientists can observe how these cells adapt their connectivity, offering insights into the mechanisms behind cognitive functions.